Ats tail gas treating process for sru and sws off gases

ABSTRACT

The present invention relates to a system and process utilizing ammonium thio sulfate solution (ATS) as the primary liquid absorption agent that is re-circulated through an SO2 Contactor/Absorber for high efficiency contacting and absorption of sulfur dioxide, SO 2  from a combustion gas stream generated by incineration of a Claus Sulfur Recovery Unit (SRU) off gas stream (often referred to as a Claus tail gas stream) and also additional SO 2  generated from incineration of additional sulfur containing streams. ATS is also re-circulated through a separate H2S Contactor/Absorber for absorption of and reaction with a Sour Water Stripper (SWS) off gas stream and additional H2S-Acid Gas (A.G.) streams to produce additional concentrated ATS. The process and equipment also provides the ability to readily switch between using ATS and ABS as the primary absorbent solution for SO 2  absorption, depending upon the concentration of SO 2  in the off gas feed streams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/351,811, filed Jan. 9, 2009, now U.S. Pat. No. 7,824,652.

FIELD OF THE INVENTION

The present invention relates to methods and systems for the treatmentof off-gasses generated by sulfur recovery processes in oil refinery andnatural gas, coal, coke and biomass gasification plant operations, andfor the recovery of sulfur and nitrogen-containing compounds generatedby the foregoing.

BACKGROUND OF THE INVENTION

Oil refinery operations, in which crude oil is processed and naturallyoccurring sulfur and nitrogen compounds are removed from the crude oil,typically produce concentrated off-gas streams of hydrogen sulfide andammonia. These concentrated tail gas streams may also be generated fromprocessing natural gas and from coal, coke and biomass gasificationoperations. Oil refinery operations are increasingly faced with tailstreams having increased contaminants, particularly sulfur and nitrogen,and there is increasing pressure on these operations to reduce therelease of these contaminants into the environment and to provide purerrefined products.

The off gas streams are typically further processed in one or moreSulfur Recovery Units (“SRU”) to recover the sulfur and to destroy theammonia. A Claus SRU is the most common type of SRU, and are generallyused as the main SRU to recover sulfur and to limit SO₂ and otheremissions. The Claus SRU off-gas is typically further treated in a TailGas Treating Unit (“TGTU”) to recover the residual un-recovered sulfur,which is generally required by environmental regulations to be used tolimit SO₂ and other emissions. The SCOT-TGTU process is typically usedfor this purpose, which involves thermal reduction of the Claus off-gasin order to convert the residual sulfur and sulfur compounds to hydrogensulfide, and recycling it back to the Claus SRU for further sulfurrecovery. Such further treatment in a SCOT-TGTU process is also neededto satisfy environmental regulations relating to the discharge of sulfurcompounds into the environment. However, the SCOT-TGTU process is anenergy- and process-intensive operation which very significantly adds tothe cost of the overall sulfur recovery operation. It also reduces thetotal sulfur recovery capacity of the main Claus SRU due to the acid gasstream being recycled back to the Claus SRU, adding to the sulfur load.

Many processes have been developed for the recovery of sulfur dioxide incombustion gas streams to meet environmental requirements, but mostrequire the addition of alkaline feed agents and generate waste productsthat are subject to additional disposal costs. For example, the processas described in U.S. Pat. No. 6,534,030 utilizes ammonium thio sulfate(ATS) as the absorption solution to absorb SO₂, producing a sulfite richATS solution which is further contacted with a feed gas containinghydrogen sulfide and ammonium to form an ammonium thiosulfate containingsolution. This sulfite-rich ATS mixture however, is not directly pHcontrolled with additional ammonia and not continuously re-circulatedthrough the SO₂ contactor with a separate re-circulating pump formaximum SO₂ recovery efficiency as is the process presented in thispatent. Further, control of ATS production volume is limited to theavailable ammonia in the feed gas stream containing hydrogen sulfide andthe ammonium. The available ammonia limits ATS production volume andlimits the amount of SO₂ that can be absorbed. Also, control of thesulfur dioxide feed gas to match available ammonia and controlabsorption pH is problematic because of variations in upstream processesthat ultimately generate the SO₂ gas feed rate. The process is limitedto the available NH₃ in the H₂S feed gas and must be controlled to meetthe required molar ratios as shown in the well known overall reactionEquation 1:

6NH₃+4SO₂+2H₂S+H₂O→3(NH₄)₂S₂O₃  Equation 1

Typically SWS gas contains approximately equal molar ratios, 1:1 ofhydrogen sulfide to ammonia which is the same ratio of sulfur to ammoniain the ATS product. U.S. Pat. No. 6,534,030 describes that unabsorbedH₂S from line 78 can be directed to an incinerator for combustion toproduce sulfur dioxide or to a Claus SRU. The total amount of SO₂ thatcan be absorbed and processed must be controlled to the SO₂:NH₃ molarratio of 4:6. Any additional SO₂ to the process can not be accommodated.Control of SO₂ gas feed to the process through line 84 to maintain therequired SO₂:NH₃ molar ratio is impractical when processing incineratedClaus SRU tail gasses. This is because both sulfur load to and sulfurrecovery efficiency of the Claus SRU is highly variable and theincinerated Claus SRU tail gas will likely exceed the limited SO₂ thatcan be accommodated by the limited ammonia feed from the SWS gas in thisATS process.

ATS solutions may be produced by the reaction of a solution of ammoniumsulfite with elemental sulfur or with sulfides including hydrogensulfide gas or sulfides or polysulfides in aqueous solution, asdescribed in Kirk-Othmer Encyclopedia of Chemical Technology. The basicprocess involves absorption and reaction of SO₂ with ammonia to producean aqueous sulfite solution. The sulfur in this sulfite solution isammonium sulfite (NH₄)₂SO₃ or ammonium bisulfite NH₄HSO₃, and usually amixture of both forming a pH buffering solution. The sulfite sulfur insolution is an oxidized form of sulfur having an oxidation valence stateof S⁺⁴. This sulfite solution is then contacted and reacted with areduced form of sulfur to produce ATS. The reduced sulfur can beelemental sulfur having a valence state of S⁰ or sulfide sulfur having avalence state of S⁻² or polysulfide which contains a mixture of sulfurhaving valence states of S⁰ and S⁻².

Many process variations utilizing different sources of sulfur andammonia, different types of contacting and reacting equipment, differentprocess flow schemes and a wide range of process conditions have beenutilized for the production of ATS and many patents have been granted onthese process variations. Most of these processes however are similar inthat they use an ammonium sulfite solution as the primary SO₂ gasabsorption solution, which is then further reacted to produce ATS.

SUMMARY OF THE INVENTION

Disclosed herein is a continuous process utilizing ammonium thiosulfatesolution (ATS) as the primary absorption solution for the highefficiency absorption, reaction and recovery of sulfur and nitrogencontaining gas components present in incinerated Claus Sulfur RecoveryUnit (SRU) off-gas streams, incinerated H₂S (Acid Gas or “A.G.”)streams, un-incinerated Sour Water Stripper (SWS) off-gas streams andun-incinerated A.G. streams commonly generated by sulfur recoveryoperations in oil refinery and gas plant operations. The processrecovers sulfur from the incinerated Claus SRU off-gas, additionalsulfur from A.G. and SWS gas and also the ammonia in the SWS gas toproduce an ATS product. The process serves as the Tail Gas Treating Unit(TGTU) for the Claus SRU to maximize total sulfur recovery and reducesulfur emissions to a minimum.

The present invention relates to a system and process utilizing ammoniumthio sulfate solution (ATS) as the primary liquid absorption agent thatis re-circulated through an SO2 Contactor/Absorber for high efficiencycontacting and absorption of sulfur dioxide, SO₂ from a combustion gasstream generated by incineration of a Claus Sulfur Recovery Unit (SRU)off-gas stream (often referred to as a Claus tail gas stream) and alsoadditional SO₂ generated from incineration of additional sulfurcontaining streams. ATS is also re-circulated through a separate H2SContactor/Absorber for absorption of and reaction with a Sour WaterStripper (SWS) off-gas stream and additional H₂S-acid gas (A.G.) streamsto produce additional concentrated ATS. The process and equipment alsoprovides the ability to readily switch between using ATS and ABS as theprimary absorbent solution for SO₂ absorption, depending upon theconcentration of SO₂ in the off gas feed streams.

Also disclosed herein is a process and system for producing ATS. Theprocess disclosed herein provides an efficient way to produce valuableATS. The final ATS product is controlled to contain a minimum of 12%nitrogen and 26% sulfur, also referred to as 60% ATS, and is primarilyused as an agricultural fertilizer solution.

In the processes disclosed herein, the use of ATS as the primaryabsorption agent has the unique advantage of absorbing lowconcentrations of sulfur dioxide from the combustion gas streams at veryhigh absorption efficiencies. The process flow equipment and processflow configuration accommodate a wide range of gas analysis, gas flowrates and flow ratios of the Claus SRU off gas, SWS off gas and A.G.feed streams that result from refinery and gas plant operations. Thishighly flexible ATS process allows for high efficiency recovery ofsulfur and nitrogen components in the feed gas streams resulting in avent stack gas that is well below environmental regulatory limits forSO₂ vented to the atmosphere. The ATS produced is a valued commercialproduct and is not a waste byproduct. The process arrangement allows forindependent control of ATS production volume over a wide range to meetmarket demand.

In the primary ATS absorption mode of operation the process disclosedherein does not produce a sulfite solution as an intermediary productfor further reaction to produce ATS. The ATS solution itself is theprimary absorption solution used to absorb sulfur dioxide in the SO2Contactor/Absorber. The resulting solution is a sulfite rich ATSsolution which is then used in a separate H2S Contactor/Absorber toabsorb and react with hydrogen sulfide and ammonia to produce additionalATS that is low in sulfite. This process uses a unique flow scheme tocontrol the pH, sulfite content and ATS concentration of two separateATS solutions being re-circulated through two separatecontactor/absorbers with a controlled recycle ATS stream between the twocontactor/absorbers to achieve optimum sulfur recovery efficiency, toaccommodate changes in ATS production volume and to provide many otheradvantages discussed herein.

The process of the invention is not limited to the available ammonia inthe SWS feed gas and does not limit the amount of SO₂ in theincineration SO₂ gas feed stream. The process is highly flexible andcontrollable using additional ammonia feed as required to controlprocess pH and provide additional ammonia to accommodate the desired ATSproduction volume. In addition to incorporating some or all the ammoniasupplied by the SWS gas feed stream the process more importantly is theTail Gas Treating Unit (TGTU) for the Claus SRU providing highefficiency emissions control and additional SRU capacity while allowingfor variable ATS production to meet market demand.

The ATS processes disclosed herein have many advantages including:utilizing some or all the ammonia in the SWS gas to supplement therequired ammonia for ATS production; increasing the Claus SRU capacityby diverting the SWS from the Claus SRU directly to the ATS-TGTU;increasing the Claus SRU capacity by diverting some or in some caseseven all the A.G. and SWS gas to the ATS unit where 100% SRU redundancyis required; increasing the Claus SRU capacity by eliminating any sulfurcontaining recycle stream back to the SRU; high sulfur recoveryefficiency to satisfy emissions regulations; the versatility to adjustthe ATS production rate to meet market demand; the versatility to adjustATS analysis to obtain optimum product specifications of assay, sulfite,alkalinity and pH; the versatility to accommodate a wide range of SO₂gas, A.G. and SWS gas stream analysis and flow rates. In addition, theprocess permits the minimization of oxidation and formation of ammoniumsulfate. The process is forgiving in that the ATS solution used toabsorb SO₂ in the SO2 Contactor/Absorber is far less solubilitydependent on pH than other ATS processes using ammonium bisulfite (ABS)as the primary SO₂ absorption solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an embodiment of the process of the invention.

DETAILED DESCRIPTION

This invention relates to an ATS tail gas treating process for recoveryof Claus SRU Incineration Gas, Acid Gas and SWS off gasses containingSO₂, H₂S, NH₃, O₂, N₂, and H₂O for the purpose of sulfur recovery,ammonia recovery, sulfur emissions control and ATS production. Theprocess provides for continuous production of ammonium thiosulfate,(NH₄)₂S₂O₃, (“ATS”), as a concentrated (60%) aqueous solution for use inagricultural and other markets.

The process uses two primary gas/liquid contactor/absorbers forcontacting, absorbing and reacting sulfur dioxide gas with ATS solutionin the SO2 Contactor/Absorber and hydrogen sulfide and ammonia gas in aseparate H2S Contactor/Absorber. ATS solution is the primary absorbersolution and is continuously re-circulated through the SO2Contactor/Absorber for SO₂ absorption and continuously re-circulatedthrough the H2S Contactor/Absorber for H₂S and NH₃ absorption. Theseparate recirculation loops have their own heat exchanger coolers andrecirculation pumps.

In addition to the separate re-circulation streams for each absorberthere is a shared recycle stream that circulates ATS between eachabsorber. This ATS recycle transports accumulated thio sulfate andammonia from the H2S Absorber/Contactor to the SO2 Contactor/Absorberand accumulated sulfite from the SO2 Contactor/Absorber to the H2SAbsorber/Contactor. If the ATS recycle stream is blocked then no ATS istransported between the two absorbers and the accumulated sulfite in theSO2 Contactor/Absorber becomes a concentrated solution of ammoniumsulfite, (NH₄)₂SO₃ and ammonium bisulfite NH₄HSO₃. This sulfite solutionis sometimes referred to as “ABS”. This ABS solution then becomes thecontacting and absorption solution for the SO₂ gas. In this case, whichis used in most prior art ATS production methods, the ABS solution isthe intermediate solution that is further reacted to produce ATS.Termination of the ATS recycle flow between the two contactor/absorbersconverts the process from the ATS absorption mode to the old establishedABS absorption mode for the standard ATS production process.

It has been determined that at high ATS production rates with highvolumes of concentrated SO₂ gas it may be advantageous to operate in theold standard ABS absorption mode. This is in part because higher SO₂absorption loads cause higher concentrations of sulfite and lower pH inthe ATS in the contact zone which can cause degradation of the ATSproduct forming some ammonium sulfate and ammonium dithionate. In thiscase the ATS absorption process is easily converted to the ABSabsorption mode and the old standard ATS production process by simplyblocking the ATS recirculation between the two contactor/absorbers.

This ability to convert to the old standard ATS production process makesthis process very versatile and adaptable to a variety of feed streamconditions and production requirements. Additionally, ABS solution canbe recovered from the SO2 Absorber/Contactor and sold as a separateproduct in addition to ATS. Also, the ABS solution can be stored andthen reintroduced to the process at a later date to increase the ATSproduction rate at that time.

In the SO2 Contactor/Absorber, SO₂ is absorbed forming a sulfite richATS solution. Even though the ATS solution is rich in sulfite it ispredominantly an ATS solution. The ATS acts to inhibit sulfite oxidationto sulfate. This enables the SO2 Contactor/Absorber to absorb SO₂ at lowconcentrations in the incineration gas stream even when there is arelatively large concentration of O₂ without appreciable oxidation ofthe contained sulfite to sulfate. The SO₂ absorption efficiency isespecially high because the ATS absorption solution is controlled to apH of about 6.0 or higher. Any unabsorbed SO₂ and SO₃ gas compounds arefurther absorbed in a SO2 Contactor/Scrubber for additional sulfurrecovery if necessary. The sulfite rich ATS is then circulated to aseparate H2S Contactor/Absorber where the H₂S and NH₃ gas compounds inthe SWS off gas and A.G. streams are absorbed by the ATS and reactedwith the sulfite component to produce additional thiosulfate.

The process as described herein refers to FIG. 1, which is the processflow schematic of the basic process equipment and flow streams of theinvention. Upstream, and not shown in the diagram, are the Claus SRU,Sour Water Stripper, Claus SRU off gas Incinerator, Incinerator WasteHeat Boiler, and the SO2 Incineration Gas Quench/Cooler. The incineratedClaus tail gas is typically cooled in a waste heat boiler and can befurther cooled by quenching the hot SO₂ gas with a cool water spray toreduce its temperature and condense excess water vapor contained in theSO₂ gas feed stream. The SO₂ gas then enters the SO2 Contactor/Absorber(T-1) through line 01.

ATS pumped from the bottom of T-1 and re-circulated through line 10contacts and absorbs the SO₂ from the SO₂ gas feed stream, whichincreases the sulfite content of the ATS, resulting in a sulfite-richATS solution at a reduced pH. Additional ammonia is supplied to T-1 tomaintain the desired solution pH through line 02A and is also suppliedby the inflow of the ammonia-rich ATS recycle stream through line 12from T-3. Additional ammonia is supplied, through lines 02B, 02C and02D, at various points to other locations in the process, to regulate pHand to supply makeup ammonia to accommodate ATS production volume ifrequired.

The accumulated sulfite in the sulfite-rich ATS is transported out ofthe T-1 system and delivered to T-3 through the ATS re-cycle return line11. ATS flow through line 12 makes up the recycle flow of low sulfiteATS from T-3 to T-1.

The gas exiting T-1 has very little remaining SO₂. Additional SO₂absorption is obtained in the SO2 Contactor/Scrubber (T-2), if required.The gas enters T-2 where it is contacted with a re-circulated stream ofdilute ATS scrubbing solution through line 14. Ammonia is added to thescrubbing solution to maintain pH through line 02B if required. The ATSin the T-2 scrubbing solution is delivered through line 15 and isdiluted by process make up water coming from the gas filter F-1 throughline 05C. The accumulated T-2 scrubber solution flows out of T-2 to T-1.

An alternative to operating T-2 with ATS in the scrubbing solution is toblock off line 15 and operate with a dilute ammonium sulfite scrubbingsolution formed from the make up water, absorbed SO₂ and ammonia. Theadvantage of incorporating ATS into the scrubbing solution is that itacts to inhibit oxidation of the sulfite component to form ammoniumsulfate. Ammonium sulfate is not a desired component in the ATS solutionbecause it reduces the contained sulfur concentration of a 60% solutionand increases the temperature at which the ATS solution will start tocrystallize. Some ammonium sulfate will usually be in the ATS solutionbut this ATS process will minimize ammonium sulfate formed fromoxidation of the sulfite component. At higher ATS production rates andin the ABS absorption mode ATS is not used in T-2. The scrubbingsolution generated by T-2 overflows to join the absorber solution inT-1.

The gas exiting T-2 passes through the Gas Filter F-1 where aerosolparticles of ammonium sulfite and ammonium sulfate are captured beforethe gas is discharged. A small amount of water is injected into the gasstream through line 05A before the gas passes through the filterelements. This water acts to dissolve the aerosol particles and preventsplugging of the filter elements by solid particles. The solution drainsfrom the filter elements and joins the balance of the required make upwater supplied through line 05B to the bottom of F-1. The accumulatedfilter solution is transferred to T-2 through line 05C.

The SWS off gas is supplied to the H2S Contactor/Absorber (T-3) throughline 03. Additional H₂S if required can be supplied from an acid gasstream split off the main acid gas feed to the Claus SRU and diverted toT-3 through line 04. Both acid gas and SWS gas diverted from beingprocessed by the Claus SRU to the ATS process reduces the sulfur feedload to the Claus SRU, and thus increases the available sulfur recoverycapacity of the Claus SRU.

Additional acid gas may be split off the main acid gas stream anddiverted from the Claus SRU to an incinerator. This will furtherincrease the available capacity of the Claus SRU while increasing theSO₂ and ATS production rate. This process versatility provides theimportant advantage of increasing or decreasing ATS production volume tomeet ATS market demand. It provides the further advantage ofsignificantly increasing the Claus SRU capacity to cover surges in totalsulfur feed or cover general increases in required total sulfur recoverycapacity if desired. This important feature provides a way for thesystem and process to accommodate the treatment of off-gasses havingvariable or increasing volume flows and concentrations of contaminants.

The H₂S and ammonia from the SWS gas and additional H₂S from acid gas iscontacted and absorbed by the ATS solution pumped from the bottom of T-3and re-circulated through line 13. The contacting ATS solution is acombination of sulfite-rich recycle ATS solution delivered from T-1through line 11, combined with a re-circulating stream of low sulfiteATS from T-3. The absorbed H₂S reacts with the sulfite component in theATS to produce additional ATS. The ammonia absorbed by the ATS isconsumed in the reactions to form the additional ATS. Any additionalammonia, if required, can be supplied through line 02C and any excessammonia is transported to T-1 with the recycle ATS through line 12.

The increased volume of ATS solution resulting from ATS produced by theprocess is taken off from either T-1 through line 08A, or T-3 throughline 08B, or a combination of both, as desired, to meet optimum sulfitecontent of the finished ATS product. The ATS is then transported tostorage through line 08.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring to FIG. 1, T-1 is the SO2 Contactor/Absorber which contactsthe SO₂ Gas stream delivered to the contactor through line 01. Thesulfur dioxide combustion gas stream comes from the Claus off-gasincinerator through a waste heat boiler and gas quench cooler upstreamof T-1 and is not shown in FIG. 1.

The SO₂ gas stream volume flow, SO₂ concentration and O₂ concentrationare determined by the upstream Claus SRU off-gas flow and acid gas flowto the Incinerator. The temperature, pressure and H₂O content of the SO₂gas stream to T-1 is determined by the upstream waste heat boiler andgas quench operations. The ATS-TGTU plant can be designed to process awide range of SO₂ concentrations and flow rates of the SO₂ gas feed,including SO₂ gas feed streams derived from the off-gas of threecatalyst bed Claus units having a sulfur recovery efficiency of greaterthan about 97% all the way to about 0% Claus sulfur recovery efficiencyfor about 100% bypass of the Claus SRU. More preferably, the Claus SRUsulfur recovery efficiency is greater than about 50% using a thermalreactor only, to greater than about 97% using three additional catalystbeds.

The SO₂ gas quench operation is utilized in most cases for 2-3 bed ClausSRU operations where there is a limited amount of SO₂ available for ATSproduction, and more water vapor in the SO₂ gas than can be accommodatedin a concentrated, 60% ATS solution. An alternative to the SO₂ gasquench operation, to condense out excess water vapor, is the use of anATS evaporator/concentrator to evaporate the excess water out of an ATSproduced at less than 60%.

A major advantage of this ATS-TGTU process is the ability to efficientlyabsorb the very low concentrations of SO₂ in the SO₂ gas derived fromincineration of Claus tail gas of an efficient three catalyst bed ClausSRU. The low SO₂ concentration is too low to be efficiently absorbed bya concentrated sulfite solution in the prior art ATS process. However,the inventors determined that the ATS absorption solution as used in thepresent invention is well suited to efficiently absorb lowconcentrations of SO₂ from the SO₂ gas for four important reasons:First, the ATS solution's pH can be maintained much higher, preferablyin the 6-7 pH range, significantly lowering the ATS solution's SO₂equilibrium vapor pressure for more efficient SO₂ absorption efficiency,versus the 5-6 pH range for a concentrated sulfite/bisulfite solution.Second, the much lower sulfite concentration in the ATS solution versusthe high sulfite concentration in a concentrated sulfite/bisulfitesolution additionally reduces the SO₂ equilibrium vapor pressure overthe ATS solution, further increasing the SO₂ absorption efficiency.Third, the concentrated ATS has a much higher solution solubility and ismuch less likely to precipitate crystals than a concentratedsulfite/bisulfite solution which will precipitate crystals if the pH isallowed to get to 6.0 or higher. Fourth, the ATS solution acts toinhibit oxidation of the sulfite component to form sulfate. Thisphenomenon is very useful in limiting ammonium sulfate in the ATSproduct when the SO₂ concentrations are low in relation to the O₂concentrations in the SO₂ gas. This phenomenon of low oxidation rates ofsulfite in ATS is possibly the result of ATS acting as a chemicaloxidation inhibitor or a result of lower oxidation rates at higher pH orfrom low sulfite concentrations available to interact with O₂ or all ofthe above.

The SO₂ gas stream 01 enters the SO2 Contactor/Absorber and flowsthrough the gas-liquid contact zone. Preferably, the SO₂ gas streamenters the SO2 Contactor/Absorber below the active contacting zone.Still more preferably, the SO₂ gas stream flows through the gas-liquidcontact zone in a preferred counter current flow direction with the ATSabsorption solution for optimum absorption efficiency, althoughco-current flow is also acceptable. The gas-liquid contacting can beaccomplished with any acceptable type of contactor equipment includingbut not limited to a packed tower, tray tower, spray tower, venturiscrubber, static mixer or bubble column.

A re-circulating stream of ATS solution is the liquid stream used tocontact and absorb the SO₂ from the SO₂ gas stream. This re-circulatingATS solution is typically pumped through a heat exchanger to cool theATS solution and remove the heat of reaction caused by the absorptionand reaction of the SO₂ with the alkalinity in the ATS solution, and thesensible heat from hot SO₂ gas, if not previously cooled in a quenchoperation. Cooling liquid, preferably water supplied by a cooling tower,is preferably used as an economical cooling media to the heat exchanger,although air cooled fin-fan heat exchangers can be used in addition toor in place of cooling water. The re-circulating ATS solution can becontrolled to temperatures below about 100° F. to about 160° F. orhigher but preferably to between about 100° F. and about 120° F. Thistemperature range conserves and retains the water in the ATS solutionand minimizes water vapor condensation in the discharged vent stack gasto form a highly visible steam plume.

The alkalinity of the ATS solution is not due to the ammoniumthiosulfate component, which is the major component in the ATS solutionand is a neutral non-buffering ammonia salt. The alkalinity isassociated with the ammonium sulfite ((NH₄)₂SO₃) component in the ATSand is the active buffering salt that reacts with SO₂ to form ammoniumbisulfite as shown in reaction Equation 2:

(NH₄)₂SO₃+SO₂+H₂O→2NH₄HSO₃  Equation 2

The total sulfite concentration in the ATS solution is the sum ofsulfite associated with ammonium sulfite plus ammonium bisulfite. Thesulfite component is defined as the negative sulfite anion, SO₃ ⁻², andits concentration in ATS solution can be determined by standardanalytical procedures and is typically reported as weight % SO₃ in theATS solution.

The pH and alkalinity of the ATS solution declines and the sulfitecontent increases as it passes through the contacting zone and absorbsSO₂. This sulfite rich ATS is continuously re-circulated through thecontacting zone to make contact with SO₂ in the gas. The sulfiteconcentration as weight percent SO₃ is maintained to less than about 10%and more than about 2% but more preferably between about 3% and about6%. The sulfite concentration is controlled by increasing or decreasingthe ATS recycle rate between T-1 and T-3 which transports sulfite-richATS out of T-1 to T-3 and low sulfite ATS from T-3 to T-1. The pH of theATS entering the contact zone is less than about 8.0 and more preferablyless than about 7.0. The pH of the ATS exiting the contact zone is lessthan about 7.0 and more preferably about 6.0. The pH and alkalinity ofthe ATS solution is maintained by the addition of ammonia to the ATSsolution through line 02A which increases the pH and alkalinity andconverts ammonium bisulfite to ammonium sulfite, as indicated inreaction Equation 3 below. Additionally the re-circulating ATS solutionpH and alkalinity in T-1 is increased by the T-3 recycle ATS, whichtransports higher pH ATS from the H2S Contactor/Absorber (T-3) throughline 12 to T-1.

NH₄HSO₃+NH₃→(NH₄)₂SO₃  Equation 3

Sulfite concentrations greater than about 10% are at risk ofprecipitating solid crystals, increased oxidation of the sulfite tosulfate and loss of ATS through decomposition reactions. Sulfiteconcentrations less than about 2% have low pH buffering capacity andlarge pH drops through the contact zone. Relatively high re-circulationflow rates through the contact zone not only increase SO₂ absorptionefficiency but also help to minimize changes in ATS solution pH andsulfite content as it passes through the contact zone.

The gas exiting T-1 contact zone has very little remaining SO₂.Additional SO₂ absorption is obtained in a second stage SO2Contactor/Scrubber (T-2), if required. Additionally, small amounts ofammonia may also be scrubbed out. The scrubber can be any suitablegas-liquid contactor but is more preferably a packed or spray tower. Ina preferred embodiment, the gas enters T-2 where it is contacted with are-circulated stream of dilute ATS scrubbing solution through line 14.Ammonia is added to the scrubbing solution to maintain pH through line02B, if required. The pH is less than about 7.0 and greater than about6.0 and more preferably about 6.6.

The ATS in the T-2 scrubbing solution is delivered through line 15 andis diluted by process make up water coming from the gas filter F-1through line 05C. The accumulated T-2 scrubber solution overflows out ofT-2 to T-1. An alternative to operating T-2 with ATS in the scrubbingsolution is to block off line 15 and operate with the dilute ammoniumsulfite scrubbing solution only. This scrubbing solution is formed fromthe normal makeup water coming from the F-1 through line 04C.

The advantage of incorporating ATS into the scrubbing solution is thatit acts to inhibit oxidation of the absorbed SO₂ as ammonium sulfite toproduce ammonium sulfate, as illustrated in reaction Equation 4:

2(NH₄)₂SO₃+O₂→2(NH₄)₂SO₄  Equation 4

The gas exiting T-2 passes through the gas filter F-1, where aerosolparticles of ammonium sulfite, ammonium bisulfite and ammonium sulfateare captured by fiber bed filter elements before the gas is discharged.A small amount of water is injected into the gas stream through line 05Bbefore the gas passes through the fiber bed filter elements. This wateracts to dissolve the aerosol particles and prevents plugging of thefiber bed filter elements by solid particles. The solution drains fromthe filter elements and joins the balance of the required make up waterin the bottom of F-1. This make up water is then transported to T-2 ifT-2 is required, or to T-1 if T-2 is not required. The total makeupwater is carefully controlled in order to control the ATS solutionspecific gravity to about 1.34 g/cc and a solution concentration ofabout 60%.

A portion of sulfite rich ATS solution is recycled to the H2SContactor/Absorber T-3 through line 11 where it joins the T-3re-circulation stream to contact H₂S and ammonia supplied by the SWSfeed through line 03 and additional H₂S in an acid gas stream suppliedthrough line 04. The H₂S gas enters the absorber preferably below theactive contacting zone and flows through the gas-liquid contact zone,preferably in a counter-current flow direction with the ATS absorptionsolution for optimum absorption efficiency. The gas-liquid contactingcan be accomplished with any acceptable type of contactor equipment,including but not limited to a packed tower, tray tower, spray tower,venturi scrubber, static mixer or bubble column. In a preferredembodiment, a packed tower is used.

A re-circulating stream of ATS solution is the liquid stream used tocontact and absorb the H₂S and ammonia from the SWS and acid gas feedstreams. The re-circulating ATS solution is typically pumped through aheat exchanger to cool and remove the heat of absorption and reactionfor high ATS production rates when the old standard ATS production usingABS in T-1 is being employed. At lower ATS production rates using ATS asthe primary SO₂ absorption solution the heat load in T-3 is very low andlittle or no cooling is required. The temperature is normally higherthan about 110° F. and more preferably about 130° F. or even higher. Thehigher temperature increases the reaction rate between the absorbed H₂Sand the residual sulfite in the ATS to help insure that there is noabsorbed but un-reacted H₂S carried to T-1 in the recycle stream 12.Un-reacted H₂S in the ATS recycle to T-1 may cause H₂S emissions in thestack vent gas if precautions are not taken to control a positivesulfite content and reaction temperature in T-3.

The sulfite rich ATS solution from T-1 is delivered to T-3 through line11, preferably at a pH of about 6.0, and contains approximately evenportions of ammonium sulfite and ammonium bisulfite. The absorption andreaction of H₂S with the sulfite components in the ATS to formadditional ATS is shown in reaction Equation 5:

2(NH₄)₂SO₃+2NH₄HSO₃+2H₂S→3(NH₄)₂S₂O₃  Equation 5

At higher ATS production rates, additional H₂S feed to T-3 is requiredfrom the acid gas supply through line 04. Sufficient H₂S is required toreact with most of the sulfite transported to T-3 from T-1. The residualun-reacted sulfite content as % SO₃ is maintained at less than about 5%and more than about 1% but preferably about 2.5%. Ammonia is supplied toT-3 in the SWS gas and additional ammonia is added if required throughline 02C to maintain the pH above about 6.5 and below about 8.0, butmore preferably about 7.3. The residual sulfite, as a mixture ofammonium sulfite and ammonium bisulfite, absorbs ammonia to becomeprimarily ammonium sulfite at a pH of 7.3. This pH is also desirable inthe ATS product to make the solution very slightly alkaline forcorrosion control.

If excess ammonia is supplied by the SWS gas, it is essentially fullyabsorbed by the ATS solution. The ammonia will be retained in the ATSsolution by converting residual un-reacted ammonium bisulfite toammonium sulfite (see reaction Equation 3) and the excess NH₃ isabsorbed and retained in solution as ammonium hydroxide which istransported to T-1 in the recycle ATS stream 12.

If excess H₂S is supplied to T-3 by the SWS gas or A.G. it will not befully absorbed, because the absorption rate is proportional to theavailable sulfite in the ATS solution, which reacts with the H₂S toproduce ATS, as shown in Equation 5 set forth above. As the sulfite isconsumed in the reaction with H₂S to produce ATS, the reduced sulfiteconcentration causes a reduction in the H₂S absorption efficiency andreaction rate. As the sulfite concentration in the ATS approaches zero,the H₂S absorption efficiency becomes very low. Unabsorbed H₂S exits thecontact zone, exits T-3 and becomes the H₂S recycle gas in line 07. Thisexcess H₂S is delivered to the Incinerator where it is combusted to formadditional SO₂, which goes to T-2 and ultimately returns to T-3 asadditional sulfite. This conversion of excess H₂S to SO₂ and its returnas sulfite to T-3 increases the sulfite concentration in T-3, whichreacts with more H₂S, reducing the excess H₂S. This creates a balanceand a somewhat stable sulfite concentration in the ATS solution in T-3.

For higher ATS production rates when additional H₂S gas flow to T-3 isrequired, the concentration of sulfite in T-3 can be controlled tonormal levels by controlling the H₂S gas flow to T-3. Lowering the H₂Sfeed rate increases the sulfite concentration in the ATS. The H₂Sabsorption and reaction rate is sufficiently high at normal sulfiteconcentrations in the 2-3% range such that only small amounts of H₂S areunabsorbed.

Additional control to maintain a desired sulfite concentration in thebottom of T-3 and reject excess H₂S may be obtained by simplycontrolling the ATS recirculation flow rate over the contact zone ifnecessary. At reduced recirculation flow there is less sulfite beingdelivered to the contact zone limiting the absorption and reaction ofavailable H₂S to form thio sulfate. This increases the sulfiteconcentration in the retained ATS in the bottom of T-3. Increasing therecirculation flow to the contact zone delivers more sulfite to thecontact zone, increases the absorption and reaction with the availableH₂S and results in lower sulfite concentrations in the bottom of T-3.

When SWS gas is the major H₂S feed stream to T-3, the ATS will likely behigh in alkalinity with excess ammonia. This excess ammonia in the ATSis in the form of ammonium sulfite and ammonium hydroxide. The excessammonia is transported to T-1 through the ATS recycle line 12, where itis utilized for SO₂ absorption.

Up to 100% of the total ammonia supplied in the SWS gas can be used tosupply up to 100% of the total ammonia required for the ATS produced.Additional ammonia is normally added to achieve desired ATS productionrates and to control pH in the process.

In cases where the SWS gas contains objectionable quantities of odorousorganic refinery compounds, the SWS gas can be diverted to the Claus SRUburner for combustion or alternatively be absorbed in a separate SWSContactor/Absorber. Additional ammonia and water may be added asrequired to form a solution of ammonium sulfide, as shown in reactionEquation 6.

2NH₃+H₂S→(NH₄)₂S  Equation 6

This solution can further be transferred to a storage vessel, where muchof the organic material separates and is taken off and recycled back torefinery feed stocks. The separated ammonium sulfide solution is thendelivered to T-3 in place of the SWS gas.

The final ATS product is taken off from either T-1 through line 08A orT-3 through line 08B, or a combination of both, as desired to meetoptimum sulfite content of the finished ATS product, and is thentransported to storage through line 08. A small amount of ammonia mayalso be added through line 02D if required to maintain optimum pH.

1. A process for continuous production of a concentrated solution ofammonium thiosulfate (ATS) comprising the steps of: (a) contacting anSO₂ containing feed gas stream with a re-circulating liquid stream ofATS solution to absorb and react with the SO₂, to produce a sulfite richATS solution; (b) contacting an H₂S containing feed gas stream with are-circulating liquid stream of ATS solution to absorb and react withthe H₂S, to produce an ATS solution having a low sulfite content; (c)recycling the sulfite rich ATS solution produced in Step (a) to contactthe H₂S containing feed gas stream of Step (b), and recycling the ATSsolution having a low sulfite content produced in Step (b) to contactthe SO₂ containing feed gas stream of Step (a), to produce additionalATS having a high sulfite content; (d) stopping the recycling in Step(c) to cause the re-circulating liquid stream of ATS solution in Step(a) to convert to an ABS solution, and contacting the SO₂ containingfeed gas stream with a re-circulating liquid stream of the ABS solutionto absorb and react with the SO₂, to produce a sulfite rich ABSsolution; (e) contacting the sulfite rich ABS solution produced in Step(d) with the re-circulating liquid stream of ATS solution in Step (b);and (f) removing the ATS solution produced in Steps (a), (b) and/or (c)to obtain a concentrated solution of ATS having optimum sulfiteconcentration and pH.
 2. The process of claim 1, wherein in Step (e) thesulfite rich ABS solution increases sulfite content of the ATS solutionproduced in Step (b).
 3. A process for increasing SO₂ absorptionefficiency and lowering SO₂ emissions in a process for continuousproduction of a concentrated solution of ATS comprising the steps of:(a) contacting an SO₂ containing feed gas stream with a re-circulatingliquid stream of ATS solution to absorb and react with the SO₂, toproduce a sulfite rich ATS solution; (b) contacting an H2S containingfeed gas stream with a re-circulating liquid stream of ATS solution toabsorb and react with the H2S, to produce an ATS solution having a lowsulfite content; (c) recycling the sulfite rich ATS solution produced inStep (a) to contact the H2S containing feed gas stream of Step (b), andrecycling the ATS solution having a low sulfite content produced in Step(b) to contact the SO₂ containing feed gas stream of Step (a), toproduce additional ATS having a high sulfite content; and (d) removingthe ATS solution produced in Step (c) to obtain a concentrated solutionof ATS having optimum sulfite concentration and pH.
 4. A process forreducing sulfate formation from oxidation of sulfite components in ATSin a process for continuous production of a concentrated solution of ATScomprising the steps of: (a) contacting an SO₂ containing feed gasstream with a re-circulating liquid stream of ATS solution to absorb andreact with the SO₂, to produce a sulfite rich ATS solution; (b)contacting an H2S containing feed gas stream with a re-circulatingliquid stream of ATS solution to absorb and react with the H2S, toproduce an ATS solution having a low sulfite content; (c) recycling thesulfite rich ATS solution produced in Step (a) to contact the H2Scontaining feed gas stream of Step (b), and recycling the ATS solutionhaving a low sulfite content produced in Step (b) to contact the SO₂containing feed gas stream of Step (a), to produce additional ATS havinga high sulfite content; and (d) removing the ATS solution produced inStep (c) to obtain a concentrated solution of ATS having optimum sulfiteconcentration and pH.
 5. A tail gas treatment unit for use with a Claussulfur recovery unit having one to three catalyst beds, said tail gastreatment unit comprising: (a) a first reactor for contacting SO₂containing gas exiting the Claus sulfur recovery unit with ATS solution,to produce sulfite rich ATS solution; (b) a second reactor forcontacting H2S containing gas with ATS solution to produce low sulfiteATS; (c) a means for transporting sulfite rich ATS produced in the firstreactor to the second reactor; and (d) a means for transporting lowsulfite ATS produced in the second reactor to the first reactor, whereinin the first reactor ATS absorbs SO₂ to produce sulfite rich ATSsolution, and in the second reactor ATS absorbs H2S to produce lowsulfite ATS solution.
 6. A process for continuous production of aconcentrated solution of ammonium thiosulfate (ATS) comprising the stepsof: (a) contacting an SO₂ containing feed gas stream with are-circulating liquid stream of ATS solution to absorb and react withthe SO₂, to produce a sulfite rich ATS solution; (b) contacting an H2Sand NH₃ containing feed gas stream with a re-circulating liquid streamof ATS solution to absorb and react with the H2S and with the NH₃, toproduce an ATS solution having a low sulfite content; (c) addingadditional NH₃ to Step (b) to contact with the recirculating liquidstream of ATS solution; (d) recycling the sulfite rich ATS solutionproduced in Step (a) to contact the H2S and NH₃ containing feed gasstream of Step (b), and recycling the ATS solution having a low sulfitecontent produced in Step (b) to contact the SO₂ containing feed gasstream of Step (a), to produce additional ATS having a high sulfitecontent; and (e) removing the ATS solution produced in Steps (a), (b)and/or (d) to obtain a concentrated solution of ATS having optimumsulfite concentration and pH.
 7. The process of claim 6, wherein Step(c) further comprises monitoring the NH₃ content of the ATS solutionproduced in Step (b) as determined by solution pH, and changing the H₂Sand NH₃ containing feed gas stream, and changing rate at which thesulfite rich ATS solution produced in Step (a) is recycled to contactthe H₂S and NH₃ containing feed gas stream of Step (b), and changingrate at which the ATS solution having a low sulfite content produced inStep (b) is recycled to contact the SO₂ containing feed gas stream ofStep (a).
 8. The process of claim 6, wherein the ATS solution having alow sulfite content produced in Step (b) contains excess NH₃, and theexcess NH₃ is recycled to Step (a).
 9. The process of claim 6, whereinthe H₂S and NH₃ containing feed gas stream is sour water stripper gas.10. A process for continuous production of a concentrated solution ofammonium thiosulfate (ATS) comprising the steps of: (a) contacting anSO₂ containing feed gas stream with a re-circulating liquid stream ofATS solution to absorb and react with the SO₂, to produce a sulfite richATS solution; (b) contacting an H₂S and NH₃ containing feed gas streamwith a re-circulating liquid stream of ATS solution to absorb and reactwith the H₂S and with the NH₃, to produce an ATS solution having a lowsulfite content; (c) recycling the sulfite rich ATS solution produced inStep (a) to contact the H₂S and NH₃ containing feed gas stream of Step(b), and recycling the ATS solution having a low sulfite contentproduced in Step (b) to contact the SO₂ containing feed gas stream ofStep (a), to produce additional ATS having a high sulfite content; (d)changing rate at which the sulfite rich ATS solution produced in Step(a) is recycled to contact the H₂S and NH₃ containing feed gas stream ofStep (b), and changing rate at which the ATS solution having a lowsulfite content produced in Step (b) is recycled to contact the SO₂containing feed gas stream of Step (a); and (e) removing the ATSsolution produced in Steps (a), (b) and/or (c) to obtain a concentratedsolution of ATS having optimum sulfite concentration and pH.
 11. Theprocess of claim 10, wherein the ATS solution having a low sulfitecontent produced in Step (b) contains excess NH₃, and the excess NH₃ isrecycled to Step (a).
 12. The process of claim 10, wherein the H₂S andNH₃ containing feed gas stream is sour water stripper gas.
 13. A processfor providing increased sulfur recovery capacity to a refinery sulfurrecovery unit operation, comprising diverting a gas stream away from thesulfur recovery unit to a reactor, wherein the gas stream is one or moreselected from the group consisting of: a sour water stripper gas streamor portion thereof and an H₂S acid gas stream or portion thereof; and aportion of the gas stream is diverted to an incinerator and incinerated;and the gas stream not diverted to an incinerator is subjected to aprocess comprising the steps of: (a) contacting an SO₂ containing feedgas stream with a re-circulating liquid stream of ATS solution to absorband react with the SO₂ to produce a sulfite rich ATS solution; (b)contacting the sour water stripper gas stream and/or the H₂S acid gasstream with a re-circulating liquid stream of ATS solution to absorb andreact with H₂S in the sour water stripper gas stream and/or H₂S acid gasstream, to produce an ATS solution having a low sulfite content; (c)recycling the sulfite rich ATS solution produced in Step (a) to contactthe sour water stripper gas stream and/or the H₂S acid gas stream ofStep (b), and recycling the ATS solution having a low sulfite contentproduced in Step (b) to contact the SO₂ containing feed gas stream ofStep (a), to produce additional ATS solution having a high sulfitecontent; and (d) removing the ATS solution produced in Steps (a), (b)and/or (c) to obtain a concentrated solution of ATS.
 14. The process ofclaim 13, wherein at least a portion of the SO₂ containing feed gasstream of Step (a) is obtained from incineration of the gas stream orportion thereof that is diverted to an incinerator and incinerated. 15.The process of claim 14 wherein no sulfur-containing compounds arereturned to the sulfur recovery unit.
 16. A process for recoveringsulfur from a refinery operation comprising: diverting away from asulfur recovery unit all or substantially all of a sour water strippergas stream and/or H₂S acid gas stream produced by the refinery operationto a reactor, and subjecting the diverted gas stream to a processcomprising the steps of: (a) contacting an SO₂ containing feed gasstream with a re-circulating liquid stream of ATS solution to absorb andreact with the SO₂, to produce a sulfite rich ATS solution; (b)contacting the sour water stripper gas stream and/or the H₂S acid gasstream with a re-circulating liquid stream of ATS solution to absorb andreact with H₂S in the sour water stripper gas stream and/or H₂S acid gasstream, to produce an ATS solution having a low sulfite content; (c)recycling the sulfite rich ATS solution produced in Step (a) to contactthe sour water stripper gas stream and/or the H₂S acid gas stream ofStep (b), and recycling the ATS solution having a low sulfite contentproduced in Step (b) to contact the SO₂ containing feed gas stream ofStep (a), to produce additional ATS solution having a high sulfitecontent; and (d) removing the ATS solution produced in Steps (a), (b)and/or (c) to recover sulfur as a concentrated solution of ATS.
 17. Theprocess of claim 16, wherein at least a portion of the SO₂ containingfeed gas stream of Step (a) is obtained by diverting a portion of thesour water stripper gas stream and/or H₂S acid gas stream to anincinerator and incinerating said portion to produce SO₂.
 18. A processfor performing maintenance on a sulfur recovery unit, comprising ceasingoperation of the sulfur recovery unit, and diverting away from thesulfur recovery unit all or substantially all of a sour water strippergas stream and/or H₂S acid gas stream produced by the refinery operationto a reactor, and subjecting the diverted gas stream to a processcomprising the steps of: (a) contacting an SO₂ containing feed gasstream with a re-circulating liquid stream of ATS solution to absorb andreact with the SO₂, to produce a sulfite rich ATS solution; (b)contacting the sour water stripper gas stream and/or the H₂S acid gasstream with a re-circulating liquid stream of ATS solution to absorb andreact with H₂S in the sour water stripper gas stream and/or H₂S acid gasstream, to produce an ATS solution having a low sulfite content; (c)recycling the sulfite rich ATS solution produced in Step (a) to contactthe sour water stripper gas stream and/or the H₂S acid gas stream ofStep (b), and recycling the ATS solution having a low sulfite contentproduced in Step (b) to contact the SO₂ containing feed gas stream ofStep (a), to produce additional ATS solution having a high sulfitecontent; and (d) removing the ATS solution produced in Steps (a), (b)and/or (c) to recover sulfur as a concentrated solution of ATS.
 19. Theprocess of claim 18, wherein at least a portion of the SO₂ containingfeed gas stream of Step (a) is obtained by diverting a portion of thesour water stripper gas stream and/or H₂S acid gas stream to anincinerator and incinerating said portion to produce SO₂.
 20. A processfor adjusting the rate of continuous production of a concentratedsolution of ammonium thio sulfate (ATS) comprising: diverting away froma sulfur recovery unit a portion of an H₂S acid gas stream produced by arefinery operation to an incinerator and incinerating the divertedportion of stream to produce SO₂ gas, and subjecting the portion of thestream not diverted to the incinerator to a process comprising the stepsof: (a) contacting an SO₂ containing feed gas stream with are-circulating liquid stream of ATS solution to absorb and react withthe SO₂, to produce a sulfite rich ATS solution; (b) contacting theportion of the H₂S acid gas stream not diverted to the incinerator witha re-circulating liquid stream of ATS solution to absorb and react withthe H₂S, to produce an ATS solution having a low sulfite content; (c)recycling the sulfite rich ATS solution produced in Step (a) to contactthe H₂S acid gas stream of Step (b), and recycling the ATS solutionhaving a low sulfite content produced in Step (b) to contact the SO₂containing feed gas stream of Step (a), to produce additional ATS havinga high sulfite content; and (d) removing the ATS solution produced inStep (c) to obtain a concentrated solution of ATS having optimum sulfiteconcentration and pH; wherein adjusting the rate is independent ofammonia available in the H₂S acid gas stream, and wherein the SO₂ gasproduced by incineration is fed to Step (a).
 21. A process forminimizing or preventing formation of crystalline solids due to pH andconcentration fluctuations in a process for continuous production of aconcentrated solution of ammonium thiosulfate (ATS), comprising thesteps of: (a) contacting an SO₂ containing feed gas stream with are-circulating liquid stream of ATS solution to absorb and react withthe SO₂, to produce a sulfite rich ATS solution; (b) contacting an H₂Scontaining feed gas stream with a re-circulating liquid stream of ATSsolution to absorb and react with the H₂S, to produce an ATS solutionhaving a low sulfite content; (c) recycling the sulfite rich ATSsolution produced in Step (a) to contact the H₂S containing feed gasstream of Step (b), and recycling the ATS solution having a low sulfitecontent produced in Step (b) to contact the SO₂ containing feed gasstream of Step (a), to produce additional ATS having a high sulfitecontent; and (d) removing the ATS solution produced in Step (c) toobtain a concentrated solution of ATS having optimum sulfiteconcentration a